More RPC/Autobuffer exception cleanup

Free channel on close, both error & non-error
MRTask2 onEx cancels local & remote
RPC handles self-calls (for ddply); use _computed flag to strip dup
calls to onEx
Dexception peels off extra dexceptions; included in TypeMap so no need
for nested RPC when reporting an exception in an RPC
ddply - print is more robust; cleanup is faster
appendablevec - when cutting off trailing empty chunks, delete potential
empty keys as well
h2ocountedcompleter onExComp prints more
Vec ChunkSummary print is more robust
Turn on ddplyBigTest
MRTask2 does not cancel

Even if completing exceptionally, allow subtaks to complete.  To hard to
get them to politely shutdown